pub struct Duration(f64);
impl From<u64> for Duration {
    fn from(s: u64) -> Self { Duration((s as f64) / (31557600 as f64)) }
}
pub trait Planet {
    fn period() -> f64;
    fn years_during(d: &Duration) -> f64 { d.0 / Self::period() }
}
macro_rules! planet {
    ($n:ident, $p:expr) => {
        pub struct $n; impl Planet for $n { fn period() -> f64 { $p } }
    }
}
planet!(Earth, 1.0);
planet!(Mercury, 0.2408467);
planet!(Venus, 0.61519726);
planet!(Mars, 1.8808158);
planet!(Jupiter, 11.862615);
planet!(Saturn, 29.447498);
planet!(Uranus, 84.016846);
planet!(Neptune, 164.79132);
